Radical Cation Diels–Alder Reaction by Photocatalysis at a Dye Sensitized Photoanode

Dye-sensitized photoelectrosynthesis cells (DSPECs) couple photodriven electron transfer between a photosensitizer and a wide-bandgap semiconductor with heterogeneous electron transfer to a substrate in an electrolyte solution. Dye-sensitized photoanodes have been utilized to drive water and alcohol oxidation reactions. The present study reports the use of a dye-sensitized photoanode in a DSPEC to drive the energy mismatched Diels–Alder reaction between trans-anethole (tA) and isoprene to form 4-(p-methoxyphenyl)-1,5-dimethylcyclohexene (1). The photoanode is comprised of fluorine-doped SnO2 (FTO) glass coated with nano-TiO2/RuP, where RuP is [Ru(2,2′-bipyridine)2(2,2′-bipyridyl-4–4′-diphosphonic acid)]2+, and it is immersed in an electrolyte solution that contains the tA and isoprene reactants. Illumination of the photoanode with AM 1.5 white light (100 mW/cm2) gives a sustained photocurrent (∼150 μA-cm–2), and HPLC and NMR analysis confirms the expected reaction to produce 1 occurs, accompanied by a cyclobutane side-product resulting from dimerization of tA. Illumination of the photoanode with monochromatic light reveals that the maximum absorbed photon to current efficiency (APCE) is ∼7.5%. Transient absorption spectroscopy indicates that the photon to current efficiency is limited by the relatively slow rate of photosensitizer regeneration (k ∼ 1 × 107 M–1 s–1). Regeneration is slow due to the low driving force (∼250 mV) for reaction of the oxidized photosensitizer (Ru3+) and tA. Optimization of the DSPEC reaction by using a blue LED matched to the RuP absorption and increasing the isoprene concentration affords nearly quantitative conversion of tA to 1.